HYDRA 



317 



regular, slender, neuro-epithelial cells which are joined into a net 

 by intercellular processes. These cells fit between the others and 

 are either sensory or motor in function thus receiving external 

 stimuli and also causing contraction of the contractile cells at 

 proper times. 



Beneath the ectoderm and embedding the bases of the cells is a 

 very thin layer of noncellular substance called mesogloea. It is 

 produced by the cell layers and serves as attachment for them, par- 

 ticularly for the fibrils of the epitheliomuscular cells. In some of the 

 other coelenterates, this layer is exceedingly thick and heavy. 



EPITHEL_IO- 



MUSCUL.AR 



CEt_l_ 



I MTER- 



STITfAU 



CEl_l_ 



NEMATOCYS" 

 CfMIDOBl-AS" 



MESOC31_OEA 



DIGESTIVE 



CEUUS 



C3l_ANO 

 CEl_U 



Fig. 113. — Cross section through the column of hydra. The central space is the 

 gastrovascular cavity. (Drawn by Titus C. Evans.) 



The inner, thicker cell layer of the wall is the endoderm which lines 

 the lumen of the gastrovascular cavity. The most conspicuous cells 

 here are the nutritive-muscular cells which are long, vacuolated struc- 

 tures attached to the mesogloea by fibrils Avhich extend in it parallel 

 to the circumference of the animal. By contraction these cells in- 

 crease the length of the animal by reducing its circumference. These 

 cells often possess flagella at the free margin and at times enguK 

 particles of partially digested food like an amoeba. It is seen then, 

 that they serve both as muscles and as digestive cells. Glandular 

 cells are also present in this layer. Being slender, they wedge them- 

 selves between the nutritive-muscular cells and secrete what is 

 probably a digestive fluid into the gastrovascular cavity. Neuro- 

 epithelial and interstitial cells are also interspersed among the 



